Solvent and process for dewaxing mineral oils



Patented Dec. 17, 1940 Donald S. McKittrick, Oakland, and Hilary-J. Henriques, Berkeley, Calif., assignors to Shell Development Company, San Francisco, 'Calif., a corporation of Delaware No Drawing. Application June 20, 1935,

Serial No. 86355 12 Claims.

This invention relates 'to a process for separating various waxy substances from mineral oil containing thesame; More particularly, it pertains to animproved diluent 'for decreasing the '5 viscosity of the oil and the solubility of the wax, while improving the solubility of the oil in the liquid phase produced when solid wax is separated from oil, whereby the wax and the oil are separated more readily than has been'posl sible heretofore.

Mineral oils naturally contain varying amounts of waxy hydrocarbons, often designated as paraffin wax or petrolatum, and hereinafter generically designated as waxes, which lat normal or elevated temperatures are dissolved in the' liquid hydrocarbons,hereinafter referred to as oil, but which solidify or become extremes.

.ly viscous at lower temperatures. If these oils are to be-used as lubricants at these reduced temperatures the solidified or highly viscous wax ,causesthe oil to resist flow. When the wax content is too low to impede flow, it may, nevertheless, cause cloudiness in the oil. It becomes necessary, therefore, to efiect'the removal ofall or some of the. wax,

This. wax probably contains aliphatic hydrocarbons' and/or naphthen'ic and other hydrocarbons; it belongs to the group of components a of petroleum which are the least soluble in selective solvents. The removal of wax may be accomplished by any of several methods. a

In accordance with one of these methods, a selective solventmedium is added. to the initial oil, and the wax-free components of the initial 3 oil are dissolved therein. This method necessitates theuse of a solvent medium which is suf-.

flciently selective to dissolve the oil at'a suitable 'dewaxing temperature without dissolving the wax. The ordinary selective solvents which are employed for example in liquid-liquid solvent extraction processes to dissolve certain types of .oils whichoare less parafiinic in nature from those which are more parafilnic in nature are not in general suitable for use as solventjmedia,

because in dewaxing processes the solvent is used to dissolve the desired oil which, in the case of extraction processes, it is desired to re.- cover in the liquid phase insoluble in the solvent.

Certain of suchselective solvents may. 'however, be employed for dewaxing purposes by adding thereto a quantity of a secondary solvent, sometimes referred to as a solubility enhancing solvent. The addition of the secondary solvent.

, improves .the miscibility of oil and selective solvent. but in most cases destroys the selectivity of the resulting mixture between wax and oil, so that the resulting. mixture of primary. selective solvent and solubility enhancing solvent is to a greatextent equivalent to .a non-selective diluent, and functions only to'reduce the viscosity of the chilled oil. We have discovered that certain oxygenated aliphatic esters which are liquid at dewaxing temperatures in the presence of oil, ,as vwell as those which become liquid upon the addition of a m solubility enhancing agent; are excellent dewaxing solvents. I Theseesters have thegeneralformula: R..C(0).O.R', where R represents a membr of the group consisting of '--R"OH,

and R, R", and 18' represent organic chain structures containing one or more .carbon, atoms, the ester containing from four to nine carbon atoms. These estersshow a highselectivity between w oil and wax, although di- 20' luted with solubility enhancing agents. -Either, R, R,. R" or R"', or several, may contain, one or more like or unlike substituents', such as: --OH, ==CO, CHO, COOH, R, COOR (where R stands foran alkyl radical), 25 Cl,' -Br, I, -NH2, --SH, =08, --CONH2, SCN, CN. Specific examples of suitable esters are: ethyl lactate, isopropyl lactate, ethyl ester of alpha-hydroxy isobutyric acid, ethyl I glycolate, diethyloxalate, dimethyl malonate 3o and diethyl malonate. Of these, the-alkyl esters of alpha hydroxy carboxylic acids are the most selective, and constitute the preferred sub-group. Ethyl lactate, CH3.C(OH).C(O).O.C2H5, is the preferred'member of this sub-group. 35

These selective solvents may be employed with any suitable enhancing solvent 'which is a good solvent both for oil and for'ithe selective solvent,

' although the solubilityenhancing solvent need not be completely miscible with the selective- I solvent or with the oil in all proportions. Any one or a mixture of many substances may be used as secondary-or solubility enhancing solvents, but they should preferably be selected from {those solvents which themselves have a'low 45 solvent power for paraflin wax. While a degree of selectivity of solubility enhancing solvent is not an absolute requirement, it was found that a greater efficiency is generally obtained when.

a fairly selective solubility enhancing solvent is 5 employed. For example butyl alcohol usually gives better results than hexane, cycle-hexane,-

or benzol. v

Numerous solubility j enhancing solvents are available, Specific examples are aliphci' alcou 1 hols, aldehydes, ethers, ketones, all containing between four and twelve carbon atoms, such as primary and secondary butyl alcohols, butyraldehyde, octylaldehyde, diethyl ketone, hexanone, diethyl ether, diethyl thioether, as well as numerous other compounds, such as carbon bisulfide, aliphatic amines, esters of lower fatty acids and lower aliphatic alcohols containing more than four and less than thirteen carbon atoms, branched chain and cyclic aliphatic hydrocarbons, aromatic hydrocarbons, etc.

While these esters may be employed with any solubility enhancing solvent, we have found that there exists a definite class of solubility enhancing solvents. whichare particularly adapted for use in connection with the above group of selective solvents. This class consists of secondary and tertiary aliphatic alcohols having from four to eight carbon atoms, and aliphatic mixed or symmetrical ethers, ketones and al'dehydes, having from four to ten carbon atoms. The aliphatic chains may be either straight or branched. Examples' of these preferred solubility enhancing solvents are: Secondary butyl alcohol, tertiary amyl alcohol, secondary hexyl alcohol, ethyl ether, lsopropyl ether, butyl ether, ethyl isobutyl ether, methyl ethyl ketone, methyl butyl ketone, ethyl isopropyl ketone, and methyl tertiary butyl ketone.

these constitute the preferred members of this subgroup. Suitable solvent mixtures may, for example, consist of from 5 to 55% of an ester of a hydroxy acid, such asethyl lactate, and from 45 to of a solubility enhancing solvent or from 10 to 65% of an ester of a di-carboxylic acid, such as diethyl oxalate, and from 35 to 90% of a solubility enhancing solvent.

The method which is the subject of our invention relies primarily on the peculiar properties of the above mixtures of solvents, which are found to create a condition in which there is a large difference between the solubility of the solid or waxy components of the initial oil and the solubility of the liquid or oil'components thereof. In one aspect, our method comprises the steps of mixing a waxy oil, such as lubricating oil, fuel oil, etc., which may be either a residual or a distillate oil, with our selective solvent and solubility enhancing solvent, and chilling the resulting mixture to a dewaxing temperature to solidify the wax, which may then be separated from the liquid portion of the system by any mechanical means, such as filtration, cold settling, or centrifuging, depending upon the character of the wax. Our process is, however, particularly suitable for filtration methods. The dewaxing temperature may, for example, be slightly below the melting point of the lowest melting wax which it is desired to remove. The pour point of the dewaxed oil may in certain cases be slightly higher than the dewaxing temperature, but by adjusting the proportions of selective solvent and solubility enhancing solvent, as described below, pour points which are as ture to a temperature within the range from 65 C. to C. or higher, and then cooling it slowly to the dewaxing temperature. When the mixture is initially warmed, it is necessary to cool the oilsolvent mixture, and this may be efiected by indirect cooling or by auto-refrigeration. very low pour points are not essential, our sol vent mixtures may often be mixed with the oil at ordinary temperatures of the order of 18 C., thereby obviating the necessity of chilling the oil.

Under certain conditions the waxwhich is precipitated according to the procedure outlined above, either with or without chilling during precipitation, may be so soft as to make its separation diflicult. It is in these cases advantageous to chill the precipitated wax during or just prior to its final removal to harden it. Subsequently the solvent mixture is removed from the separated liquid by any means, such as distillation.

To be filterable or otherwise separable from the oil-solvent phase, the wax particles must be of suitable form, size and strength and/or hardness. We have found that if iri precipitating the wax a very small amount of liquid oil is separated out from the oil with'the wax crystals, clusters of crystals are formed and the separation of wax from the solution of oil and solvent is facilitated. The amount of oil necessary for this purpose depends upon the size of the initial wax crystals, which, in turn, depends among other things, upon the nature of the wax and the rate of chilling. In processes in which the crystals are extremely small and the wax content of the initial oil is high, as much as 1.0% of the main body of the oil may be desirable. In other cases, as when the crystals are larger, and/or when less wax is present, lesser amounts of oil may be desirable, and it is frequently practical to permit substantially no oil to be separated out with the wax. Since the separation of an excess quantity of oil reduces the yield of dewaxed oil, it is desirable to control the amount of the oil which is separated with the wax, as described below.

A convenient method of controlling this amount of oil is to regulate the concentration of the solubility enhancing solvent in the solvent mixture. The necessary dilution ratio, 1. e., the ratio of the combined solvent mixture to the initial oil, is generally determined by the viscosity of the oil and the solvent mixture and by the selectivity of the solvent mixture at the dewaxing temperature, and is made as low as possible with a view of conserving the solvent.- When using the new solvent mixtures of our invention, the dilution ratio may be much lower than the ratio employed with the known solvent mixtures. For example, we have obtained excellent results when using a dilution ratio of 2:1, and even lower ratios, such as lzl may be employed, although we prefer to employ ratios of about 4:1. At very low temperatures ratios as high as 8:1 may be desirable.

For a given dilution ratio, the greater the con-' centration of the selective solvent component of the dewaxing mixture, the more complete will be' the precipitation of wax, and extremely low pour points may often be obtained. However, since the selective solvent has a relatively low solvent power for the oil, the oil-solvent system will in this case form two liquid phases, in addition to the solid wax phase, and the yield of the dewaxed oil will be extremely low. When an excess of Where solubility enhancing solvent is employed, the oilsolvent system will exist as a homogenous liquid phase, and both pour points and yields of dewaxed oil will be increased. Between these concentrations of solubility enhancing solvents, there is a composition of the'combined solvent mixture forming a transition point between the liquidsolid and liquid-liquid-solid phase systems. The

transition composition will, of course, depend upon several factors, such as the dilution ratio, the dewaxing temperature, the, specific solvent employed,,and the character of the oil, but may be easily determined empirically for any givensituation.

having specific gravities' We'have found that'when the volume concentration of the solubility enhancing solvent in the mixed dewaxing-solvent is lower than about 10% below the concentration corresponding .to the transition composition, the yields ofdewaxed oil are reduced'quite out of proportion to the improvement in the/pour point.

found that the. most efiicient dewaxing operations Moreover, we have are those which are carried out by employing dewaxing solvent mixtures having compositions near the transition composition, i. e., in which the corn centration of the solubility enhancing solvent fall in the range from 10% below to 20% above the transition concentration. Using greater quantities of solubility enhancing agent results in greater yields, but the pour points of the dewaxed oil will often be undersi'rably high.

Such solvent mixtures as are used in accordance with the present invention may, for example, at temperatures of about F. to F'.,-have com-, plete or substantially complete solubility for oil I i between 0.90 and 0.94 and refractive indices 11.13 9 between 1.50. and 1.52, and substantially no temperatures, 1. e., not more than a few tenths of .one per cent of wax will be dissolved at these solvent power for wax which is solid at the said temperatures; such an oil may, for example, have a viscosity of between 50 and 55 sec. Say. Univ. at

- teristic may, of course. be used also with oils of.

viscosities.

210 F. Mixtures of solvents having this characdifferent refractive indices, specific gravitiesor The dewaxing process may also be carried out in several steps, the composition of the solvent mixture being regulated, and the process being carried out as described in our Patent No. 2,126,- 493.

The yields of dewaxed oil of a given pour point and the filtration rates-can often be further increased by adding a small amount, generally between 0.2% and 1%, of a pour point reducing substance, such as 'tetrastearyl glucoserpentae- .rithrite tetrastearate, distearyl picene, crackedresidues, metallic 'soaps, etc., to the oil, preferably prior to chilling. Most of the'pour point reducer separates from the oil with the wax, so that it is not normally eifective to lower the pour point of the filtrate, but is effective'to aid in the formation of fllterable wax crystals.

The following examples illustrate the superior results obtainable with our improved solvent mixtures. For comparison, the same oil was dewaxed with several of our solvent mixtures, and with acetone-benzol mixtures, the latter mixture being regarded as standard for dewaxing mir ral oils.

Example I.Several samples of a. Ventura distillate having a pour point of 95 F. were each diluted with four volumes of various solventmixtures, heated to produce a homogeneous system,

gradually cooled, and filtered at between 0 C.-

and 1 C. The diluent compositions and yields are given in per cent by volumes. The results are shown in Table I:

'(ontainod 10% toluene.

With a dilution ratio of 1 to 4at 0 C., the following compositions of solvent dewaxing mixtures corresponded to the transition point between one and two liquid phase systems: 52% .diethyl oxalate, 48% secondary butyl alcohol; 38% ethyl Iactate, 62% secondary butyl alcohol; 72% acetone, 28% benzol. It will be noted that in each case the pour points became lower and the yields dropped ofi as:less solubility enhancing agent was employed. The decrease in yield became'more marked when the quantity of the solubility enhancing agent became less' than corresponds to the transition point for the solvent in question,

but the further lowering of the pour pohit was not as marked. It is also apparent that the efliciency of the dewaxing operation, expressed in terms of yields and pour points of dewaxed oil was higher in the experiments in which our dewaxingv solvents were employed than in the ex-,

periments in which the acetone benzol mixtures were used.

Example II.The distillate described in Example I was similarly dewaxed, but the filtration was carried out at between 19 C. and -20 C.

,The results are shown in Table II:

Table II Diluont FX Yield of f 1 dewnxed g Selective solvent a tm ml of oil Ethyl lactate: Sec. butyl alcohol: Percmf 1'. l IO'TZ...v '90 85 7.5 2 83 --l2.5 3 83 l5 4 81 -15 Contained'lll'Z. toluene.

With dilution ratios of 1 to 4 at -20 C., the following compositions of solvent dewaxing mixtures corresponded to the transition-point between one and two.liquid phase systems: 20%

ethyl 1mm, 80% secondary butyl alcohol: 55% acetone, benzol. The superiority of our diluent mixtures is apparent from the above table,

which shows that dewaxed oils with considerably lower pour points can be produced without mixtures. With the acetone-benzol mixtures, 0 F. was the lowest pour point obtainable.

It will be noted that our new dewaxing mixtures are particularly advantageous for low temperature dewaxing operations, 1. e.. those carried out below 0 0., such 10 C., -20 C., or lower.

The above examples are given only for the purpose of illustrating the eificiency of our new solvents, and not by way of limitation. Thus, the oil-solvent ratio may be varied within wide limits, and other specific selective solvents and/or solubility enhancing solvents may be employed. The dewaxing may be carried out either as a continuous or as a batch process. By the term effective quantity, as used in the claims, we mean a quantity ofsolvent mixture which will maintain sufficient oil in solution to produce its substantially complete separation from the wax which is precipitated, and which will produce an oil-solvent mixture of the desired viscosity.

We claim as our invention:

1.- A selective solvent dewaxing mixture for petroleum oils, comprising an alkyl ester .of an aliphatic hydroxy carboxylic acid, said ester having from four to nine carbon atoms, and a solubility enhancing solvent having a high solvent power for said ester and for petroleum oil, the ester and the solubility enhancing solvent being in such proportion that, at 10 F., their mixture has a substantially complete solvent power for oil having a specific gravity between 0.90 and 0.94, and a refractive index between 1.50 and 1.52, and substantially no solvent power for wax which is solid at -l0 F., the ester being liquid at' l0" F. in the presence of said petroleum oil and the solubility enhancing solvent.

2. A selective solvent dewaxing mixture for petroleum oils, consisting of between 5 and 55% ethyl lacatate and between 45 and 95% of a nonprimary aliphatic alcohol having from four to eight carbon atoms.

3. A selective solvent dewaxing mixture for petroleum oils, consisting of between 5 and 55% ethyl lactate and between 45 and 95% secondary butyl alcohol.

4. In the process of manufacturing low pour point lubricating oil from wax bearing mineral oil substantially as described, the step of treating the oil with a liquid solvent to separate solid wax from the oil, said solvent comprising an ester of the formula: R.C(O) .O.R'-, where R represents a member of the group consisting of R".OH, -R"(O).O.R"', R".O.(O) R', and

RIP.OR-III,

and R, R", and R'" represent organic chain temperature in the presence of the oil. a

5. In the process of manufacturing low pour point lubricating oil from wax-bearing mineral oil substantially as described, the step of treating the oil with a liquid solvent mixture to separate solid wax from the oil, said solvent mixture comprising an ester of the formula: R.C(O).O.R'.

where R represents a member oi the group con sisting of -R".OH, -R-"(O) .O.R',

and -R".O.R"', and-R, R", and R represent organic chain structures, said ester having from four to nine carbon atoms, and a solubility enhancing solvent having a high solvent power for said ester and for the oil, at a temperature sufliciently low to solidify the wax, the ester bein liquid at said temperature in the presence of said mineral oil and the solubility enhancing solvent in such proportions that the resulting solvent mixture has substantially complete solvent action on the oil and substantially no solvent action on the wax at the temperature of the process.

6. A process for dewaxing mineral oils which comprises incorporating with the oil an alkyl ester of an alpha aliphatic hydroxy carboxylic acid, said ester having from four to nine carbon atoms and being liquid in the presence of the oil under conditions of the process, chilling the oil, and separating the wax from the oil.

7. The process of dewaxing mineral oils which comprises mixing said oil'with a liquid. selective dewaxing solvent comprising an alkyl ester of an aliphatic hydroxy acid, said ester having from four to nine carbon atoms, precipitating solid Wax from the resulting oil-solvent mixture in the presencev of a solubility enhancing solvent which is a good solvent for oil and for said ester, said ester liquid at the dewaxing temperature in the presence of said mineral oil and the solubility enhancing solvent the -ratio of the ester to the solubility enhancing solvent being such that at the dewaxing temperature the resulting solvent mixture has substantially complete solvent power for the oil and substantially no solvent action on the wax, and mechanically removing the precipitated wax from the oil-solvent mixture.

8. The process according to claim 7 in which the concentration of the solubility enhancing solvent in the solvent mixture is not smaller than 10% below the quantity necessary to prevent the formation of two liquid phases at the temperature at which the wax is removed from the oil-solvent mixture.

9. The process according to claim '7 in which the ester is ethyl lactate.

10. The process according to claim '7 in which the ester is ethyl lactate and the solubility enhancing solvent is a non-primary butyl alcohol.

11. The process for dewaxing mineral oils which comprises mixing the oil with at least an equal volume of a solvent mixture containing between 5 and 55% ethyl lactate and between 45 and 95% secondary butyl alcohol, chilling the resulting mixture to precipitate wax, and removing the precipitated wax from the oil-solvent mixture by mechanical means.

12. The process for dewaxing mineral oils which comprises mixing the oil with at least an equal volume of a solvent mixture containing between 10 and 65% diethyl oxalate and between 35 and 90% secondary butyl alcohol, chilling the resulting mixture to precipitate wax, and removing the precipitated wax from the oil-solvent mixture by mechanical means.

DONALD S. MCIH'I'IRICK.

HILARY J. HENRIQUES. 

